Spiral VS bunched wires?

[tonyh]

Hi, Im getting to the final stages of building my Barracuda and thinking about my coil.
The shell I have bought will only take a bunched wire coil, I was wondering if coil builders out there have any performance comparision between a flat spiral and a bunched coil? or are they as good as each other.
Thanks

[baum7154]

Though I have not built one, the spiral coil is generally faster than the typical bunched coil. The intent of all Fast coils is to allow the detector to 'look' sooner after the coil has quieted. This is largely attributable to distributed capacitance in the coil and it's feedline. In a bunch wound coil each wire may run closely parallel with 2 to 6 or 7 other wire turns. The spiral coil geometry minimizes capacitance by keeping a winding widely spaced from the other windings typically through the use of thickly insulated wire laid down in a spiral on a flat plane. This means a given winding turn is only in proximity to the previous turn and to the turn that follows. There is a lot of information on spiral coils on this forum if you take a little time to search.

Good Luck!

Dan

[bbsailor]

Though I have not built one, the spiral coil is generally faster than the typical bunched coil. The intent of all Fast coils is to allow the detector to 'look' sooner after the coil has quieted. This is largely attributable to distributed capacitance in the coil and it's feedline. In a bunch wound coil each wire may run closely parallel with 2 to 6 or 7 other wire turns. The spiral coil geometry minimizes capacitance by keeping a winding widely spaced from the other windings typically through the use of thickly insulated wire laid down in a spiral on a flat plane. This means a given winding turn is only in proximity to the previous turn and to the turn that follows. There is a lot of information on spiral coils on this forum if you take a little time to search.

Dan

Dan,

Although what you said is true, there is one more thing to consider. If you shield the coil, a spiral coil may have more capacitance than a bundle wound coil. Therefore, consider the added capacitance tradeoff when you need to shield the coil as whatever you gain in lower capacitance with a spiral wound coil may be given up when you shield it with more surface area between the coil and shield.

The true test is to measure the coil self resonance of both style coils with a shield and the one with the highest self resonance is potentially faster.

I hope this helps.

bbsailor

[baum7154]

Good point on the shielding adding capacitance, I was addressing only the coil geometry. As your 'Making a Fast Pulse Induction Monocoil' paper points out it is important to provide physical separation of the shield from the coil using a good dielectric material such as PTFE aka teflon. I tend to use 6 mm or 1/4" spacing as a general rule. Having not constructed a spiral coil I have not dealt with the shielding of it either. Is it easier to shield a bunch wound coil with less resulting capacitance than a spider coil? If I were shielding a spider coil I'd be tempted to try some 1/4" thick urethane packing foam sheet on top and bottom of the spiral as a spacer and then wrap it with self adhesive cloth hospital tape before applying the graphite shield material. Air bubbles in the foam are a great dielectric too. I agree that in the end the final word is the self resonant frequency of the coil, shield, and feed line as a system.

Thanks for the comment!

Regards'

Dan

[baum7154]

Correction to the type of foam I'd like to try on spacing the shielding of the spiral coil...NOT Urethane foam...Polyethylene foam is what I meant. It is commonly used as packing material for electronics etc. Comes in sheets of varying thickness. Usually white in color but sometimes black or pink as well. Easily cut with a razor blade and a good amount of rigidity. Great dielectric properties!

Sorry for the confusion.

[baum7154]

Having not constructed a spiral coil I have not dealt with the shielding of it either. Is it easier to shield a bunch wound coil with less resulting capacitance than a spider coil? If I were shielding a spider coil I'd be tempted to try some 1/4" thick urethane packing foam sheet on top and bottom of the spiral as a spacer and then wrap it with self adhesive cloth hospital tape before applying the graphite shield material. Air bubbles in the foam are a great dielectric too. I agree that in the end the final word is the self resonant frequency of the coil, shield, and feed line as a system.

Thanks for the comment!

Regards'

Dan[/QUOTE]

Oh my it was late last night when I replied with the quote above. Please substitute SPIRAL for the 'spider' coils references . I meant to stick to the SPIRAL coil topic. Unfortunately I have been building spider coils recently and for a long time and had that stuck in my brain and fingertips, so I inadvertently typed SPIDER when I meant SPIRAL. It didn't help that the first three letters of each are the same! Autopilot took over and I crrrrrrashed!

OOPS!
Dan